MOS transistors, such as MOSFET or IGBT, include a control structure by which the component is controllable, such as adapted to be switched on and off. This control structure includes a gate electrode of an electrically conducting material and being insulated against a body zone of the MOS transistor by a dielectric layer, or the gate dielectric. The body zone is arranged between a source zone and a drain zone (which are also referred to as emitter zone and collector zone in an IGBT), where in power MOS transistors a drift zone is arranged between the drain zone and the body zone, with the drift zone being lowly doped compared to the drain zone.
The gate electrode serves for controlling a conducting channel in the body zone along the gate dielectric. Dependent on the component this channel is an accumulation or inversion channel. The MOS transistor is controlled by applying an electrical potential (gate potential) to the gate electrode or by applying an electrical voltage (gate voltage) between the gate electrode and the source zone, respectively. The absolute value of the gate voltage to be applied for effecting a conducting channel in the body zone and the polarity of this gate voltage is dependent on the type of the transistor.
In power semiconductor components having a voltage blocking capability of several 100V up to several kV a significant voltage is present across the gate dielectric when the component blocks or is switched off. The dielectric strength of this gate dielectric layer influences the voltage blocking capability of the overall component. In general the strength of the gate dielectric layer may be increased by increasing its layer thickness. However, higher gate voltages for controlling the component are required when increasing the layer thickness.
The gate dielectric layer is, e.g., a semiconductor oxide, such as silicon oxide in a silicon component. At field strengths that lie above about 6 MV/cm a Fowler-Nordheim-tunneling-current over the oxide layer sets in. The breakthrough field-strength is about 10 MV. In use of a component field-strengths higher than 4 MV/cm should be avoided in order to avoid degradation. This maximum allowable field-strength limits the allowable gate voltage, and therefore limits the charge density that can settle in in the conducting channel along the gate dielectric in the body zone.
Further, the gate dielectric may degrade due to temperature induced mechanical stress, cosmic radiation, or injection of hot charge carriers, the latter occurring during avalanche breakthrough of a MOSFET, e.g.
For these and other reasons there is a need for the present invention.